While the topic was almost certainly prompted by the AOL/Goodmail controversy, the debate was more theoretical, about whether a system in which the sender pays makes sense for e-mail. To make it brief, Dyson's point (which I tend to agree with) is that we should try anything and everything that seems to hold the promise of stopping the spam problem, and market forces will correct the system should things go wrong. O'Brien's main argument revolved around a few ideas: making people pay contributes to the digital divide; an "artificial market" creates perverse incentives (ISPs would have no incentive to develop their own anti-spam systems if they're making money from the likes of Goodmail); and that market forces can't necessarily work their magic in an environment where people have a high cost of switching ISPs (giving up their e-mail addresses). I'm sure I'm leaving something out here, but you get the gist.

All in all, a very satisfying and interesting debate. Considering the lengths to which anti-AOL/Goodmail activists have gone (accusing AOL of lying and deliberately blocking legitimate e-mail), I was surprised, but pleased, that it was so cordial and thought-provoking.